This invention relates to high energy electrochemical power cells. More particularly, it relates to cells having an oxidizable anode material and a liquid active cathode material and a solid current collector.
Recently there has been a rapid growth in portable electronic products requiring electrochemical cells to supply the energy. Examples are calculators, cameras and digital watches. These products, however, have highlighted the deficiencies of the existing power cells for demanding applications. For example, digital watches were developed using the silver oxide cell, and although these watches have become popular, it is now generally recognized that the least developed component of the digital watch system is the power cell. In particular, the energy density of the silver cells is such that thin, stylish watches with reasonable operating life are difficult to make. Additionally, these cells have poor storage characteristics, low cell voltages, and leakage problems.
In an effort to develop a cell that addresses one or more of the foregoing problems, substantial work has been done with cell chemistries using a lithium anode. The cathode and electrolyte material consisting of a solvent and solute vary. Indeed the literature is replete with examples of lithium anode cells with different cathodes and electrolytes. The electrical characteristics of these cells such as energy per unit volume, called energy density; cell voltage; and internal impedance vary greatly.
Among all of the known combinations of lithium anodes with different cathodes and electrolytes, those believed to have among the highest energy density and lowest internal impedance use certain inorganic liquids as the active cathode depolarizer. This type of cell chemistry is commonly referred to as "liquid cathode".
Early liquid cathode cells used liquid sulfur dioxide as the active cathode depolarizer as described in U.S. Pat. No. 3,567,515 issued to Maricle, et. al. on Mar. 2, 1971. Since sulfur dioxide is not a liquid at room temperature and at atmospheric pressure, it proved to be quite a difficult chemistry with which to work. More importantly, sulfur dioxide cells are unsafe for most consumer applications due to their propensity to explode under certain circumstances.
A major step forward in the development of liquid cathode cells was the discovery of a class of inorganic materials, generally called oxyhalides, that are liquids at room temperature and also perform the function of being the active cathode depolarizer. Additionally, these materials may also be used as the electrolyte solvent. Liquid cathode cells using oxyhalides are described in U.S. Pat. No. 3,926,669 issued to Auborn on Dec. 16, 1975, and in British Pat. No. 1,409,307 issued to Blomgren et al. on Oct. 18, 1975. At least one of the oxyhalides, thionyl chloride (SOCl.sub.2), in addition to having the general characteristics described above, also provides substantial additional energy density.
As described in the Auborn and Blomgren patents, the anode is lithium metal or alloys of lithium and the electrolyte solution is an ionically conductive solute dissolved in a solvent that is also an active cathode depolarizer.
The solute may be a simple or double salt which will produce an ionically conductive solution when dissolved in the solvent. Preferred solutes are complexes of inorganic or organic Lewis acids and inorganic ionizable salts. The requirements for utility are that the salt, whether simple or complex, be compatible with the solvent being employed and that it yield a solution which is ionically conductive. According to the Lewis or electronic concept of acids and bases, many substances which contain no active hydrogen can act as acids or acceptors or electron doublets. In U.S. Pat. No. 3,542,602 it is suggested that the complex or double salt formed between a Lewis acid and an ionizable salt yields an entity which is more stable that either of the components alone.
Typical Lewis acids suitable for use in the present invention include aluminum chloride, antimony pentchloride, zirconium tetrachloride, phosphorus pentchloride, boron fluoride, boron chloride and boron bromide.
Ionizable salts useful in combination with the Lewis acids include lithium fluoride, lithium chloride, lithium bromide, lithium sulfide, sodium fluoride, sodium chloride, sodium bromide, potassium fluoride, potassium chloride and potassium bromide.
The double salts formed by a Lewis acid and an inorganic ionizable salt may be used as such or the individual components may be added to the solvent separately to form the salt. One such double salt, for example, is that formed by the combination of aluminum chloride and lithium chloride to yield lithium aluminum tetrachloride.
In addition to an anode, active cathode depolarizer and ionically conductive electrolyte, these cells require a current collector.
According to Blomgren, any compatible solid, which is substantially electrically conductive and inert in the cell, will be useful as a cathode collector since the function of the collector is to permit external electrical contact to be made with the active cathode material. It is desirable to have as much surface contact as possible between the liquid cathode and the current collector. Therefore, a porous material is preferred since it will provide a high surface area interface with the liquid cathode material. The current collector may be metallic and may be present in any physical form such as metallic film, screen or a pressed powder. Examples of some suitable metal current collectors are provided in Table II of the Auborn Patent. The current collector may also be made partly or completely of carbon. Examples provided in the Blomgren Patent use graphite.
Electrical separation of current collector and anode is required to insure that cathode or anode reactions do not occur unless electrical current flows through an external circuit. Since the current collector is insoluble in the electrolyte and the anode does not react spontaneously with the electrolyte, a mechanical separator may be used. Materials useful for this function are described in the Auborn Patent.
Although the varied cells described in the Blomgren and Auborn patents may be feasible, much of the recent interest is in cells using thionyl chloride as the active cathode depolarizer and electrolyte solvent. This results from thionyl chloride's apparent ability to provide greater energy density and current delivery capability than other oxyhalide systems. Yet even though thionyl chloride cells have proven to be the best performer among the oxyhalides, they have not lived up to expectations on energy density or internal impedance. Furthermore, the thionyl chloride cell is equally if not more dangerous than the sulfur dioxide cell. As a result, all known efforts to commercialize cells using this chemistry have failed.